The present invention relates to an improved magnetoresistive head and a method and an apparatus for initializing the same, and more particularly to an improved magnetoresistive head being improved in a high reproduced output and operational stability, and being used for storing information into a magnetic recording medium and reproducing the stored information, wherein the information is stored as a direction of magnetization of the magnetic medium.
Recently, a sensitivity of the magnetic sensor and a high density of the magnetic recording have underscored the importance of the developments of the magnetoresistive sensor and the magnetoresistive head.
The magnetoresistive sensor and the magnetoresistive head operate by reading out external magnetic field signals based upon variations in resistance of the sensor or detector portion made of a magnetic material. Since a relative speed of the head to the magnetic medium does not depend upon the reproduced output, the magnetoresistive sensor can show a high sensitivity whilst the magnetoresistive head can have a high output even at a high density magnetic recording.
In Physical Review Letters vol. 61, p. 2472, 1988, it is disclosed that a magnetoresistive effect due to a spin dependent scattering can be observed in multilayer in the form of a magnetic synthetic lattice.
The above magnetoresistive effect is a giant magnetoresistive effect, much larger than the conventional magnetoresistive effect.
In the Japanese laid-open patent publication. No. 2-61572, it is disclosed that the magnetoresistive effect is caused due to the spin dependent scattering. A magnetic sensor has at least two ferromagnetic layers separated by an intermediate layer. Those layers are made of materials such that the electron scattering depending upon the spin direction is caused on the boundary interfaces.
In Physical Review B vol. 43, p. 1297, 1991 and Japanese laid-open patent publication No. 4-358310, it is disclosed that at least two ferromagnetic layers are separated by a non-magnetic metal layer and further an anti-ferromagnetic layer is provided adjacent to one of the two ferromagnetic layers so that the magnetization direction of the one ferromagnetic layer is pinned, whilst the magnetization direction of the other ferromagnetic layer is free to rotate in accordance with an externally applied magnetic field for causing a large variation in resistance thereof. This structure is a spin valve structure which shows a giant magnetoresistive effect.
FIG. 1 is a schematic view illustrative of one of the conventional magnetic heads utilizing the giant magnetoresistive effect. A magnetic sensing portion 50 is provided for sensing magnetic signals. The magnetic sensing portion 50 comprises a pair of first and second ferromagnetic layers 51 and 53 separated by a non-magnetic metal layer 52 and an anti-ferromagnetic layer 54 adjacent to the second ferromagnetic layer 53. The anti-ferromagnetic layer 54 provides an exchange coupling which pins the magnetization of the second ferromagnetic layer 53 in a direction "A". The second ferromagnetic layer 53 serves as a pinned layer wherein the magnetization is pinned. The first ferromagnetic layer 51 is a free layer wherein the magnetization is free to rotate. If the magnetization of the second ferromagnetic layer 53 is pinned in the direction "A" which is parallel to a direction "B" of the magnetic field signal externally applied whilst the magnetization direction of the first ferromagnetic layer 51 serving as the free layer is perpendicular to the direction "B" of the signal magnetic field, then a large linear response and a wide dynamic range are obtained. The magnetization of the first ferromagnetic layer 51 is free to rotate whilst the magnetization of the second ferromagnetic layer 53 is pinned, whereby an angle between the magnetization directions of the first and second ferromagnetic layers 51 and 53 is varied. This variation in angle between the magnetization directions of the first and second ferromagnetic layers 51 and 53 causes a variation in resistance of the magnetic sensing portion 50. This variation in resistance of the magnetic sensing portion 50 is detected.
Not illustrated in FIG. 1, a permanent magnetic layer or an anti-ferromagnetic layer is provided at edge portions of the above layers for generating a longitudinal bias magnetic field which causes a single domain of magnetization of the first ferromagnetic layer 51.
The above conventional magnetic head utilizing the giant magnetoresistive effect has the following problems. The first problem is that the magnetization of the second ferromagnetic layer 53 is pinned in a direction which is parallel to the signal magnetic field but which is perpendicular to magnetization directions of the magnetic layers forming the magnetic head. FIG. 2 is a fragmentary cross sectional elevation view illustrative of the conventional magnetic head comprising an inductive portion for magnetic recording and a magnetic sensing portion utilizing the giant magnetoresistive effect for reproducing signals. The conventional magnetic head has a magnetic sensing portion 50 exhibiting the giant magnetoresistive effect. The conventional magnetic head also has a pair of top and bottom shields 58 and 57 for improvement in density of magnetic recording in a bit direction. The conventional magnetic head also has a pair of top and bottom poles 61 and 59 for recording signals into a magnetic medium. The conventional magnetic head also has a permanent magnet 56 for causing a stable single domain of magnetization of the first ferromagnetic layer 51. The top shield 58 serves not only the shield layer but also the bottom pole 59.
In the high frequency range, it is necessary for recording signals into the magnetic medium or reproducing the signals that the easy axes of the top and bottom shields and the top and bottom poles are perpendicular to the direction "B" of the magnetic field signal. Those magnetic layers are grown or heat-treated under application of the magnetic field for control of the magnetic anisotropy so that the magnetic layers have desired easy axes orientation.
The permanent magnet layer 56 is provided for causing the stable single domain of magnetization of the first ferromagnetic layer 51. The magnetic anisotropy of the permanent magnet layer 56 is directed perpendicular to the direction "B" of the magnetic field signal.
As described above, the easy axis of the top and bottom shields 58 and 57 is perpendicular to the direction "B" of the signal magnetic field. The easy axis of the top and bottom poles 61 and 59 is also perpendicular to the direction "B" of the signal magnetic field. The magnetization direction of the permanent magnet layer 56 is also perpendicular to the direction "B" of the signal magnetic field. On the other hand, it is required that the magnetization direction "A" of the second ferromagnetic layer 53 be pinned in the direction parallel to the direction "B" of the signal magnetic field.
In order to pin the magnetization of the second ferromagnetic layer 53 by use of the anti-ferromagnetic layer, it is required to rise the temperature up to a Neel temperature of a film on growth or the anti-ferromagnetic layer under the magnetic field for polarization.
If, however, the above temperature rising process is carried out during the growth of the films having the giant magnetoresistive effect, then it is possible that the easy axis of the bottom shield 57 is disturbed and further that the pining of the magnetization of the second ferromagnetic layer 53 is disturbed in the control processes for the magnetic anisotropy of the top shield 58, and the top and bottom poles 61 and 59 after the layers having the giant magnetoresistive effect have been grown.
On the other hand, if the temperature rising process for pinning the magnetization of the second ferromagnetic layer 53 and subsequent polarization process are carried out after all of the layers forming the magnetic head have been grown, then undesired influences are provided onto the easy axes and magnetic properties of the top and bottom shields 58 and 57, the top and bottom poles 61 and 59 and the permanent magnet layer 56 and the other layers forming the magnetic head.
Further, it is possible that, during the processes of the magnetic head, the magnetic properties of the magnetic material of the magnetic head are varied. Since the second ferromagnetic layer 53 is different in magnetization direction from the other magnetic layers, it is very difficult to carry out an initial setting again in the final state of the magnetic head.
The second problem with the magnetic head utilizing the giant magnetoresistive effect is that a magnetic field is caused by a detection current flowing through the magnetoresistive films so as to weaken the pinned magnetization of the second ferromagnetic layer 53.
As illustrated in FIG. 2, a pair of electrodes 55 are provided for applying a detection current to the magnetoresistive films for detecting signal magnetic fields. The symmetry of the reproduced signal waveforms largely depends upon the direction of the detection current. Since the magnetization of the second ferromagnetic layer 53 is pinned by the anti-ferromagnetic layer 54, a static magnetic coupling is generated between the first and second ferromagnetic layers 51 and 53 whereby the magnetization direction of the first ferromagnetic layer 51 is varied from a direction perpendicular to the direction "B" of the signal magnetic field. As a result, a dynamic range to the signal magnetic field is made narrow.
If the detection current is applied in such a direction "C" that the direction of the magnetic field generated in the first ferromagnetic layer 51 by the detection current is anti-parallel to the direction of the static magnetic field of the second ferromagnetic layer 53 pinned by the anti-ferromagnetic layer 54 whereby the static magnetic field by the second ferromagnetic layer 53 is weakened to improve the symmetry of the reproduced signal waveform.
The magnetic field caused by the detection current weakens the pinning of the second ferromagnetic layer 53 whereby the magnetization of the second ferromagnetic layer 53 is gradually disturbed.
In the above circumstances, it had been required to develop a novel magnetic head utilizing a giant magnetoresistive effect and a method of initialization thereof free from the above problems.